In traumatology has been accepted generally the method to use surgical devices (osteosynthesis devices), where the parts of broken bone are held together (immobilized) by means of plate-like, perforated osteosynthesis plates, which are fixed to bone by screws, to immobilize the fractured bone during the healing period.
The demands of such surgical devices are very high because they must have sufficient mechanical properties, be compatible with the tissues and they must permit undisturbed osseous union. Usually such surgical devices have been manufactured of austenitic steel, titan or of other metals or metal alloys, which fulfill the demands of tissue environment. Generally such conventional biostable surgical devices are removed at a separate subsequent operation, when the healing of the fracture has proceeded by means of the growth of the bone tissue far enough.
The use of metallic surgical elements causes, however, many kinds of problems. Many metals and metal alloys corrode in tissues causing inflammation and infections, which are problematic and are eliminated only after removing the surgical device. The corrosion is often found in contact points of plates and screws.
Because of the large difference in stiffness between bone (Youngs modulus E=6-20 GPa) and metals (E=100-200 GPa), rigid metallic fixation prevents the rapid proliferation of primary callus and deprives the bone of the normal stress-pattern. The long-term result of stress-protection is a mechanically inferior bone structure in the region of the plate because of osteoporosis and atrophy. Refracture is therefore a possibility after the removal of the plate.
The possibilities to use other than metallic materials as surgical devices have been studied extensively during last years. On the basis of these studies polymers and polymer composites seem to be in this respect promising materials.
Polymers and composites which are useful as surgical implant-materials can be classified to biostable and totally or partially resorbable. Accordingly surgical devices can be classified to biostable, resorbable and to at least partially resorbable.
The reaction, which biostable polymeric surgical devices cause to tissue, is minimal and the implant retains its form and properties practically unchanged even during long periods of time (typically years). Such a surgical device has been defined e.g. in a Swiss Pat. No. 618 866.
Totally resorbable and at least partially resorbable polymeric or composite surgical devices retain their tissue-supporting properties certain periods of time (typically weeks of months) and they are gradually degraded biologically into tissue compatible components which are absorbed by living tissues and replaced by healing tissues. In the case of partially resorbable devices a non-supporting encapsulated remnant of device can remain in tissue.
When one uses biostable polymeric surgical devices, whose modulus is the same order of magnitude as the modulus of bone, one can diminish the disadvantages of osteoporosis and metallic corrosion remarkably. However, these devices in many cases must be removed at a separate operation. When one manufactures surgical devices of totally or at least partially resorbable polymers or composites it is possible to avoid the removal operation. Therefore the at least partially resorbable surgical devices are the best one alternative because of the following reasons:
(B 1) At the early stage of healing of fracture the at least partially resorbable device preserves the required bone immobilization. At the later stage the device decomposes gradually and the stresses are transferred gradually to the healing bone. This prevents osteoporosis. PA0 (2) The mechanical properties of the surgical devices which are manufactured of organic material or composite can be regulated nearer to the corresponding properties of bone than the properties of metallic implants. PA0 (3) Totally or at least partially resorbable surgical devices do not need the removal operation which means substantial economical and human advantages.
The most studied in surgery applicable resorbable polymers are polyglycolides, polylactides and their copolymers. Their manufacturing and applications as sutures etc. are disclosed in several patents e.g. in U.S. Pat. No. 2,668,162, U.S. Pat. No. 2,676,945, U.S. Pat. No. 3,297,033, U.S. Pat. No. 3,463,158, U.S. Pat. No. 3,636,956 and Can. Pat. No. 808 731. U.S. Pat. No. 3,297,033 discloses the principle to use polyglycolide fibers in conjunction with other structures as prosthetic devices. This principle is applied in French Pat. Appl. No. 78 29878 which discloses the resorbable surgical device of polylactide which in reinforced with resorbable polyglycolide fibers. An application of this invention discloses a perforated surgical device which is screwed to bone with stainless steel screws (P. Christel et al., Biomaterials 1980, p. 271).
U.S. Pat. No. 3,463,158 discloses the resorbable sheet of polyglycolic acid which is fixed to the bisected ends of femurs of the hind legs of rabbits by means of polyglycolic nails which are driven through the holes in the polyglycolic plate and bone.
FIG. 1 shows schematically the conventional surgical osteosynthesis device fixed to bone. According to FIG. 1 the bone 1, which includes the fracture 2, is immobilized by means of the osteosynthesis plate 3 which goes beyond the fracture 2 and which is fixed to the bone by means of nails or screws 4. The number of screws is typically between 6-8.
Also other publications which handle applications of at least partially resorbable materials as surgical devices disclose fixation techniques which apply perforated plates.
The fixation of perforated plates by screws or nails is suitable for metals because the compressive stresses caused by screwing or nailing to the plate as a rule do not exceed the yield strength of the metal. So the metallic materials behave in these conditions elastically.
Surgical devices which are manufactured of polymers or polymer composites are viscoelastic of their mechanical behavior. Therefore they do not have a clear region of elastic behaviour. Therefore the compressive stresses caused by nailing or screwing around the fixing hole cause in these devices continuous deformation (creep), which leads to the enlargening of the hole which causes the loosening of fixation and/or the formation of fractures around the hole. Additionally polymeric materials are notch sensitive and therefore the fixation holes act as harmful points of stress concentrations which also promotes the formation of fractures around the holes (P. Christel et al., Biomaterials, 1980, p. 271).
Additionally the screw fixation of plates causes to the screw-bone boundary strong local torsional stresses, which means that this boundary is the weakest point in such constructions.